The Cosmic Landscape

by Leonard Susskind

  At the other end of the country, New Jersey is the home of two of the world’s great centers for theoretical physics. Princeton, with its university Physics Department and the Institute for Advanced Study, is first and foremost, but twenty miles north, in New Brunswick, is another powerhouse—Rutgers University. Michael Douglas is one of Rutgers’s star attractions. Like Witten, he is both a brilliant physicist and a serious mathematician. But more important for this tale, he is a bold explorer of the Landscape. Douglas has set himself the task of studying the statistics of the Landscape rather than the detailed properties of individual valleys. He uses the laws of large numbers—statistics—to estimate which properties are most common, what percentage of valleys lies at different altitudes, and what the likelihood is that a valley that can support life exhibits approximate supersymmetry. While he prefers to use the term statistical approach instead of Anthropic Principle, it’s probably fair to say that Douglas is on the anthropic side of the divide.

  Cosmologists are equally split on the issue. Jim Peebles of Princeton University is the “grand old man” of American cosmology. Peebles has been a pioneer in every aspect of the subject. In fact in the late 1980s he was one of the very first people to suspect that cosmological data indicated the existence of something like a cosmological constant. In discussing the problems of cosmology with him, I was struck by his rather automatic acceptance that many features of the universe could be explained only by some kind of anthropic reasoning.

  Sir Martin Rees, the British Astronomer Royal, is an all-out enthusiast for the Landscape, the megaverse, and the Anthropic Principle. Martin is Europe’s leading cosmologist and astrophysicist. Many detailed arguments that I have used to motivate the Anthropic Principle I learned from him and from the American cosmologist Max Tegmark.

  Andrei Linde and Alexander Vilenkin you have already met. Like Rees and Tegmark, they are firmly in the anthropic Landscape camp. Linde has expressed his opinion: “Those who dislike anthropic principle are simply in denial. This principle is not a universal weapon, but a useful tool, which allows us to concentrate on the fundamental problems of physics by separating them from the purely environmental problems, which may have an anthropic solution. One may hate the Anthropic Principle or love it, but I bet that eventually everyone is going to use it.”

  Stephen Hawking is Martin Rees’s colleague at Cambridge University, but I have no doubt that his views are very much his own. Here is a quote from a lecture Stephen gave in 1999: “I will describe what I see as the framework for quantum cosmology, on the basis of M theory. I shall adopt the no boundary proposal, and shall argue that the Anthropic Principle is essential, if one is to pick out a solution to represent our universe, from the whole zoo of solutions allowed by M theory.”

  So it seems that Stephen and I finally agree on something.

  But not all cosmologists agree. Among the best-known Americans in the field, Paul Steinhardt and David Spergel are vehement foes of anything that smells vaguely anthropic. Steinhardt, whose feelings are more or less representative, says he hates the Landscape and hopes it will go away. But like Maldacena he can find no way to get rid of it. From Steinhardt’s writings (in “The Edge Annual Question—2005,” at www. edge.org): “Decades from now, I hope that physicists will be pursu-ing once again their dreams of a truly scientific ‘final theory’ and will look back at the current anthropic craze as millennial madness.”

  Alan Guth, the father of Inflation, is a fence sitter. Alan is a thorough believer in the populated Landscape. Indeed, it was he who coined the term pocket universe. But not being a string theorist, he takes a wait-and-see attitude toward the discretuum—in other words, he is less committed to the proposition that the number of possible vacuum environments is exponentially large. As for the Anthropic Principle, I suspect Alan is a closet believer. Whenever I see him I say, “Well Alan, have you ‘come out’ yet?” He invariably answers, “Not yet.”4

  I’ve saved for last my old friend David Gross. David and I have been good friends for forty years. During that time we have fought and argued incessantly, sometimes fiercely, but always with great respect for each other’s opinions. My guess is that we will become two crusty old curmudgeons, battling to the very end. Maybe we already are.

  David is, without doubt, one of the world’s greatest living physicists. He is best known as one of the principal architects of Quantum Chromodynamics, i.e., the dynamics of hadrons.5 But more important for this story, he has long been one of the most senior generals in the army of string theorists. In the mid-1980s, while a professor at Princeton, David and his collaborators Jeff Harvey, Emil Martinec, and Ryan Rohm created a sensation when they discovered Heterotic String Theory. This new version of String Theory looked much more like the real world of elementary particles than any previous version. Moreover, at about the same time Ed Witten (also at Princeton) was busy with his collaborators—Andy Strominger, Gary Horowitz, and Philip Candelas—inventing Calabi Yau compactification. When the two came together, the world of physics gasped—the results looked so realistic that it seemed only a matter of months until a definitive, final, unique theory of elementary particles would be in hand. The world held its breath—and held its breath and turned blue.

  Fate was not kind. The more time that passes, the more it becomes clear that the Princeton enthusiasm was, at best, premature. But David has never given up the hope that the silver bullet will turn up and make the earlier enthusiasm justified. Myself? I suspect that, in the end, the Heterotic theory will turn out to be a very important component of Rube’s great machine. Its resemblance to the Standard Model is impressive. But I would also guess that it is not the only component. Fluxes, branes, singularities, and other features may expand the Heterotic Landscape far beyond what the authors of the theory originally imagined.

  Gross, as I said, is an extremely formidable intellectual opponent, and he is very opposed to the Anthropic Principle. Although his reasons are more ideological than scientific, they are important to discuss. What bothers him is an analogy with religion. Who knows? Maybe God did make the world. But scientists—real scientists—resist the temptation to explain natural phenomena, including creation itself, by divine intervention. Why? Because as scientists we understand that there is a compelling human need to believe—the need to be comforted—that easily clouds people’s judgment. It’s all too easy to fall into the seductive trap of a comforting fairy tale. So we resist, to the death, all explanations of the world based on anything but the Laws of Physics, mathematics, and probability.

  David, along with many others, expresses the fear that the Anthropic Principle is like religion: too comforting, too easy. He fears that if we begin to open the door, even a crack, the Anthropic Principle will seduce us into a false belief and stop future young physicists from searching for the silver bullet. David eloquently quotes Winston Churchill’s 1941 address to students at his own school: “Never, ever, ever, ever, ever, ever, ever, give up. Never give up. Never give up. Never give up.” But the field of physics is littered with the corpses of stubborn old men who didn’t know when to give up.

  David’s concern is very real, and I don’t mean to minimize it, but I also think it’s not as bad as he says. I don’t for a moment worry about the younger generation lacking the moral fiber to avoid the trap. If the populated Landscape is the wrong idea, we (or perhaps I should say, they) will find it out. If the arguments that indicate the existence of 10500 vacuums are wrong, string theorists and mathematicians will discover it. If String Theory itself is wrong, perhaps because it is mathematically inconsistent, it will fall by the wayside and, with it, the String Theory Landscape. But if that does happen, then as things stand now, we would be left with no other rational explanation for the illusion of a designed universe.

  On the other hand, if String Theory and the Landscape are right, with new and improved tools we may locate our valley. We may learn about the features in neighboring locales—including the inflationary ledge and steep downhill
approach. And, finally, we may confirm that the rigorous use of mathematics leads to many other valleys, in no special way distinguished from ours except by their inhospitable environment. David has honest concerns, but to shun a possible answer because it runs counter to our earlier hopes is itself a kind of religion.

  Gross has another argument. He asks, “Isn’t it incredibly arrogant of us to presume that all life must be just like us—carbon based, needful of water” and so on. “How do we know that life can’t exist in radically different environments?” Suppose, for example, some strange forms of life could evolve in the interior of stars, in cold dust clouds of interstellar space, and in the noxious gases that surround giant gas planets like Jupiter. In that case, the Ickthropic Principle of the codmologists would lose its explanatory power. The argument that life’s need for liquid water explains the fine-tuning of the temperature would lose its force. In a similar vein, if life can form without galaxies, then Weinberg’s explanation of the smallness of the cosmological constant also loses its force.

  I think that the correct response to this criticism is that there is a hidden assumption that is an integral part of the Anthropic Principle, namely: the existence of life is extremely delicate and requires very exceptional conditions. This is not something that I can prove. It is simply part of the hypothesis that gives the Anthropic Principle its explanatory power. Perhaps we should turn the argument upside down and say that the success of Weinberg’s prediction supports the hypothesis that robust intelligent life requires galaxies, or at least stars and planets.

  What are the alternatives to the populated Landscape paradigm? My own opinion is that once we eliminate supernatural agents, there is none that can explain the surprising and amazing fine-tunings of nature. The populated Landscape plays the same role for physics and cosmology as Darwinian evolution does for the life sciences. Random copying errors, together with natural selection, are the only known natural explanation of how such a finely tuned organ as an eye could form from ordinary matter. The populated Landscape, together with the rich diversity predicted by String Theory, is the only known explanation of the extraordinary special properties of our universe which allow our own existence.

  This is a good place for me to pause and address a potential criticism that might be leveled against this book, namely that it lacks balance. Where are the alternative explanations of the value of the cosmological constant? Aren’t there any technical arguments against the existence of a large Landscape? What about other theories besides String Theory?

  I assure you that I am not hiding the other side of the story. Throughout the years many people, including some of the most illustrious names in physics, have tried to explain why the cosmological constant is small or zero. The overwhelming consensus is that these attempts have not been successful. Nothing remains to report on that score.

  As for serious mathematical attempts to debunk the Landscape, I know of only one. The author of that attempt is a good mathematical physicist, and as far as I know, he still believes his criticism of the KKLT construction (see chapter 10). The objection involves an extremely technical mathematical point about special Calabi Yau spaces. Several authors have criticized the criticism, but by now it may be irrelevant. Michael Douglas and his collaborators have found many examples that avoid the problem. Nevertheless, an honest assessment of the situation would have to include the possibility that the Landscape is a mathematical mirage.

  Finally, as for alternatives to String Theory, a well-known one is called Loop Gravity. Loop Gravity is an interesting proposal, but it is not nearly as well developed as String Theory. In any case even its most famous advocate, Lee Smolin, believes that Loop Gravity is not really an alternative to String Theory but may be an alternative formulation of String Theory.

  As much as I would very much like to balance things by explaining the opposing side, I simply can’t find that other side. Opposing arguments boil down to a visceral dislike of the Anthropic Principle (I hate it) or an ideological complaint against it (it’s giving up).

  Two specific arguments have been the subjects of recent popular books by well-known physicists, but both have failed in my view. I’ll take a moment to explain why.

  The Laws of Nature Are Emergent

  This is a favorite idea of some condensed-matter theorists who work on the properties of materials made of ordinary atoms and molecules. Its principal proponent is the Nobel Prize winner Robert Laughlin, who describes his ideas in his book A Different Universe.6 The idea at its core is the old “ether theory” that maintains that the vacuum is some special material. The ether idea was popular in the nineteenth century, when both Faraday and Maxwell tried to think of electromagnetic fields as stresses in the ether. But after Einstein the ether fell into disrepute. Laughlin would like to resurrect the old idea by picturing the universe as material with properties similar to superfluid helium. Superfluid helium is an example of a material with special “emergent” properties, properties that reveal themselves (emerge) only when huge numbers of atoms are assembled in macroscopic amounts. In the case of liquid helium, the fluid has amazing superfluid properties such as flowing without any friction. In a lot of ways, superfluids are similar to the Higgs fluid that fills space and gives particles their properties. Roughly speaking Laughlin’s view can be summarized by saying that we live in such a space-filling material. He might say it even more strongly: space is such an emergent material! Moreover, he believes that gravity is an emergent phenomenon.

  One of the main themes of modern physics is that emergent phenomena have a kind of hierarchical structure. Little collections of molecules or atoms group together to form bigger entities. Once you know the properties of these new entities, you can forget where they came from. The new entities, in turn, combine and cluster into new groups of even larger size. Once again you can forget where they came from and group them into yet bigger groups until the whole macroscopic material is explained. One of the most interesting properties of these systems is that it doesn’t matter exactly what you begin with. The original microscopic entities don’t make any difference to the emergent behavior—the material always comes out with the same large-scale behavior—within limits.7 For this reason Laughlin believes there is no point in looking for the fundamental objects of nature, since a wide variety of basic objects would lead to the same Laws of Physics—gravity, the Standard Model, and so on—in the large-scale world. In fact there are all kinds of “excitations” in materials that do resemble elementary particles but are really collective motions of the underlying atoms. Sound waves, for example, behave as though they were made of quanta called phonons. Moreover, these objects sometimes behave uncannily like photons or other particles.

  There are two serious reasons to doubt that the laws of nature are similar to the laws of emergent materials. The first reason involves the special properties of gravity. To illustrate, consider the properties of superfluid helium, although any other material would do as well. All sorts of interesting things take place in superfluids. There are waves that behave similarly to scalar fields and objects called vortices that resemble tornadoes moving through the fluid. But there is no kind of isolated object that moves around in the fluid and resembles a black hole. This is not an accident. Black holes owe their existence to the gravitational force described by Einstein’s General Theory of Relativity. But no known material has the characteristics that the General Theory of Relativity ascribes to space-time. There are very good reasons for this. In chapter 10, where I dealt with black holes, we saw that the properties of a world with both quantum mechanics and gravity are radically different from anything that can be produced with ordinary matter alone. In particular, the Holographic Principle—a mainstay of current thinking—seems to require totally new kinds of behavior not seen in any known condensed-matter system. In fact Laughlin himself illustrates the point by arguing that black holes (in his theory) cannot have properties, such as Hawking radiation, that practically everyone else believes them to have. />
  But suppose one found an emergent system that had some of the features that we want. The properties of emergent systems are not very flexible. There may be an enormous variety of starting points for the microscopic behavior of atoms, but as I said, they tend to lead to a very small number of large-scale endpoints. For example, you can change the details of helium atoms in many ways without changing the macroscopic behavior of superfluid helium. The only important thing is that the helium atoms behave like little billiard balls that just bounce off one another. This insensitivity to the microscopic starting point is the thing that condensed-matter physicists like best about emergent systems. But the probability that out of the small number of possible fixed points (endpoints) there should be one with the incredibly fine-tuned properties of our anthropic world is negligible. In particular, there is no explanation of the most dramatic of these fine-tunings, the small but nonzero cosmological constant. A universe based on conventional condensed-matter emergence seems to me to be a dead-end idea.

  Natural Selection and the Universe

  Lee Smolin has attempted to explain the very special properties of the world—the Anthropic properties—by a direct analogy with Darwinian evolution—not in the general probabilistic sense that I explained earlier but much more specifically.8 To his credit Smolin understood early that String Theory is capable of describing a tremendous array of possible universes, and he attempted to use that fact in an imaginative way. Although I feel that Smolin’s idea ultimately fails, it is a valiant effort that deserves serious thought. The gist of it follows: